The invention relates to an arrangement and a method for active cell-balancing of an electrical energy storage device that comprises multiple storage cells.
There is a growing need for energy storage systems that can be used in numerous ways both in stationary and also in mobile applications. Relevant examples thereof are inter alia emergency electrical current systems or electric/hybrid vehicles.
The electrical energy storage device comprises for example multiple storage elements that are embodied as rechargeable batteries and comprise storage cells that are embodied as battery cells. The individual battery cells of the batteries are connected in series and/or in parallel in order to obtain performance and/or energy data that are adjusted to suit the respective application.
In so doing, in the case of individual battery cells, differences with respect to the electrical characteristics (for example voltage, capacity and internal impedance) are observed inter alia on the basis of production fluctuations. In one battery, the individual battery cells are therefore generally not identical with respect to their aging state and their electrical performance.
Generally, in the case of battery cells that are connected in series in the battery, the discharging procedure is terminated as soon as one of the battery cells has achieved their discharging voltage limit. This also applies for the charging procedure of the battery. The charging procedure stops as soon as the charging voltage limit of one of the battery cells is achieved. The cell that has the smallest capacity and is often also referred to as the weakest cell is therefore the first cell to become completely charged or discharged. Consequently, the battery or the battery module is always only as good as the weakest cell of said battery or said battery module.
In order to be able to provide a battery having uniformly loadable cells and furthermore in order also to prevent that the individual battery cells additionally age differently as a result of different voltage states, methods are known for so-called cell-balancing, said methods having the goal of balancing the voltages of the individual battery cells.
If the voltage and the charge is not balanced in such a manner, the charge state of the different cells drifts during operation as a result of slightly different capacities and slightly different self-discharges of the cells or modules.
With respect to the methods for cell-balancing, active cell-balancing differs from passive cell-balancing, wherein, in the case of active cell-balancing using a battery management system, charge balancing between the storage modules is performed in such a manner that the energy of the cells that are too highly charged and that comprise the highest cell voltage is distributed to cells that have a too low charge and have a low cell voltage. In the case of a passive cell-balancing, those cells that have the highest battery voltage are discharged in a purposeful manner until all cells achieve the identical voltage. The discharging procedure is generally performed using resistors, wherein the electrical energy that is to be discharged is converted into heat.
EP 2400622 describes inductive cell-balancing between the cells of an energy storage device, wherein the energy storage device has an inductor by way of which it is possible using an arrangement of switches and diodes to exchange energy between the cells.
EP 2385605 describes an active charge balancing circuit. The charge balancing circuit comprises an inductive storage element and a circuit arrangement that can connect the inductive storage element in response to a control signal to a multiplicity of cell connections so as to perform the charge balancing.
US 2012/0194134 describes a method for charge balancing in a charge storage device arrangement that comprises a multiplicity of charge storage cells that are connected in series, and a charge balancing circuit. Moreover, an inductive charge balancing circuit is described that is configured so as to draw energy from a storage cell and to feed this energy selectively back into a storage cell or the entire charge storage device arrangement.